(Please, no guesses...if you don't know, don't spread or start misinformation)

Question (in 2 parts):

1) The typical Bo is 6 ft or 2 meters long. Has this always been the case or has the Bo undergone a change in its length?

2) During the period of formalization of Bo techniques (18th to 19th century), the average Okinawan male was about 5 ft tall. If the Bo was 6 feet long, proper technique would dictate that the Bo should be about 1 foot taller than its weilder. I'm average height (5'9") so shouldn't I use a Bo closer to 7 feet rather than 6 feet in length?

Good question, right?

The old Chinese answer is the length of a real fighting stick should be up to the user's eye-brow, whatever his height; this was in the days when it was actually used for fighting and not demonstrations. Lift the weapon up a couple of inches and it will fall just between your foot and the top of your head.

Reason? If the stick is higher than you by a foot, your techniques will be restricted largely to upper parries, straight thrusts and horizontal strikes. High speed vertical swings and twirls will mean constantly hitting the ground and thus require constant adjustments of hand positions -- something you cannot afford to do in a real fight.

There was no change of length; only students (tall or short) having to use what was available in the dojo, regardless of the students' height. But if you were to use the stick as a personal weapon as an everyday survival tool, carrying a 7 feet stick around is not pratical. A stick up to your eyebrow is just comfortably right for carrying around and fighting, especially if you need to enter a door.

i am 5'9 as well. I use a rokkushaku bo. 6ft. but be careful when just using the term bo. Some people will get technical and say what kind? hanbo , sanbo, tanbo,jo, even a little 6inch yawara stick is considered a bo in some peoples eyes.
the only instance ive seen of someone using a longer staff would be my sensei. He is about 6ft4. He uses a rokkushaku bo go sun. 6ft5 inch bo. ive used his bo before and it was a bit awkward.